While amorphous silicon and low-temperature polysilicon are widely used as a semiconductor layer of a display device such as a liquid crystal display or an electroluminescence display, various oxide semiconducting materials have been developed in the context of increase in the display size, realizing high precision, reduction in power consumption and the like.
An oxide semiconducting material may be, for example, an indium gallium and zinc oxide (IGZO), which has features such as high electron mobility and small leakage current. Besides IGZO, oxide semiconducting materials of various compositions such as an indium gallium oxide (IGO), a gallium zinc oxide (GZO), a zinc tin oxide (ZTO), an indium zinc tin oxide (IZTO) and an indium gallium zinc tin oxide (IGZTO) have been considered as oxide semiconducting materials that have better features.
In general, an oxide semiconducting material is formed as a thin film on a substrate such as glass using a film forming process such as a sputtering technique. Then, it is etched using a resist or the like as a mask to form an electrode pattern. This etching process may be a wet type (wet technique) or a dry type (dry technique), where the wet technique uses an etching liquid.
Among the oxide semiconducting materials, oxides containing at least zinc and tin are excellent in chemical resistance, and thus are stable even when they are exposed to various chemicals and gases during the film forming process and the etching process of other peripheral materials. On the other hand, however, oxides containing at least zinc and tin have a problem of having difficulty in fabrication by wet etching and the like.
When a pattern of an oxide semiconducting material is formed by wet etching, the etching liquid is required to have the following performances (1)-(5).
(1) It has a preferable etch rate (E.R.).
(2) Fluctuation in the etch rate as the oxide dissolves in the etching liquid is small. In other words, the etching liquid is stable and durable for long-term use and has a prolonged chemical solution life.
(3) It does not generate a precipitate when dissolving an oxide.
(4) It does not corrode peripheral materials such as wiring.
(5) The pattern shape (taper angle, linearity, residue removal performance) of the oxide semiconductor after the etching is good.
The etch rate of an oxide semiconducting material is preferably 10 nm/min or more, more preferably 20 nm/min or more, and still more preferably 30 nm/min or more. At the same time, it is preferably 10000 nm/min or less, more preferably 5000 nm/min or less, and still more preferably 2000 nm/min or less. Especially, it is preferably 10-10000 nm/min, more preferably 20-5000 nm/min, and still more preferably 30-2000 nm/min. When the etch rate is 10-10000 nm/min, high production efficiency can be maintained and the etching operation can be performed stably.
Furthermore, the oxide concentration in the etching liquid increases with etching. It is desirable that the decrease or change in the etch rate due to this is small. When the etching liquid is used to etch an oxide semiconductor layer, this is extremely important for realizing efficient industrial production.
Moreover, when a precipitate is generated in an etching liquid having an oxide semiconducting material dissolved therein, the precipitate may remain on the substrate as a residue after the etching treatment. This residue may induce generation of voids, adhesion failure, leakage or disconnection in the subsequent film forming process. As a result of which, characteristics as a display device could be deteriorated.
In addition, when a precipitate is generated in an etching liquid having an oxide semiconducting material dissolved therein, this precipitate may clog up a filter that is provided for circulating the etching liquid, whose replacement is cumbersome and may lead to high cost.
Therefore, even if the performance as an etching liquid is still remaining, the etching liquid needs to be discarded before the generation of such precipitate, resulting in shorter duration of use of the etching liquid and increase in the cost of the etching liquid. Additionally, cost for disposing waste liquid also increases.
For example, when zinc oxide is etched using an etching liquid containing oxalic acid, there is a major problem that an zinc oxalate precipitate as a solid matter. In a general etching liquid containing oxalic acid, a precipitate is generated when a concentration of the dissolved zinc becomes about 10 mass ppm (Comparative Examples 1 and 2).
Accordingly, it is desirable that a precipitate is not generated when zinc is dissolved in an etching liquid. A specific amount of dissolved zinc is preferably 10 mass ppm or more. More preferably it is 100 mass ppm or more, and particularly preferably it is 1000 mass ppm or more.
Although there is no upper limit, in order to perform a safe and stable etching operation, it is preferably 5000 mass ppm or less, more preferably 4000 mass ppm or less, and particularly preferably 3000 mass ppm or less.
Examples of a wiring material generally used for a display device such as a liquid crystal display include copper (Cu), aluminum (Al), molybdenum (Mo) and titanium (Ti). Since the etching liquid may possibly make contact with these wiring materials upon etching the oxide semiconducting material, corrosion of these wiring materials should preferably be ignorable or small. Specifically, the etch rate of the wiring material is preferably 3 nm/min or less, more preferably 2 nm/min or less, and still more preferably 1 nm/min or less.
The pattern shape of the oxide semiconductor after the etching specifically has a taper angle (an angle between the etched surface at the edge of the semiconductor layer and the surface of the underlying layer) of preferably 10°-80°. FIG. 5 is a schematic view showing cross-sectional observation of the semiconductor layer after the etching treatment. A semiconductor layer 2 and a resist 1 are laminated on an underlying layer 3, where the semiconductor layer 2 is patterned by using the resist 1. Here, the angle between the etched surface at the edge of the semiconductor layer and the surface of the underlying layer is referred to as a taper angle 4. The taper angle is more preferably 15°-75°, and particularly preferably 20°-70°. When the taper angle is larger than this range, there is a problem that the coverage with a layer laminated thereon will be poor. When the taper angle is smaller than this range (see FIG. 3), the linearity (the linear shape of the edge of semiconductor layer vertically seen from above) tends to be poor (see FIG. 4).
Furthermore, the pattern shape of the oxide semiconductor after the etching has a maximum linearity error of preferably 0.2 μm or less, more preferably 0.15 μm or less and still more preferably 0.1 μm or less. When the linearity is poor, an error of the width of the semiconductor layer is caused, which is unfavorable. FIG. 6 is a schematic view showing the top surface of the semiconductor layer vertically observed from above after the etching treatment and peeling off the resist. The view shows, in order from the left, the underlying layer 5, the tapered portion 6 of the semiconductor layer formed by the etching treatment and the semiconductor layer 7. The maximum value of an error 9 of the linearity (in the figure, indicated by a dotted line) of the border line 8 at the edge of the semiconductor layer patterned by the etching treatment is referred to as the “maximum linearity error”.
Moreover, no residue (remainder or precipitates of oxide, etc.) is preferably generated on the underlying layer that has been removed of the etched oxide semiconductor layer (see FIG. 2).
As an etching liquid for ZTO, an etching liquid containing hydrochloric acid and nitric acid as primary components is known from Patent Literature 1.
Furthermore, Patent Literature 2 describes that ZTO can be etched with an aqueous organic acid solution such as oxalic acid or an aqueous inorganic acid solution such as those of halogen-based or nitric acid-based inorganic acid.
Moreover, Patent Literature 3 discloses an etching liquid characterized by a composition containing (a) oxalic acid, (b) a naphthalene sulfonate condensate or a salt thereof, (c) at least one of hydrochloric acid, sulfuric acid, water-soluble amine and salts thereof, and (d) water, which is used for etching an indium oxide-containing film.
Patent Literature 4 discloses an etching liquid characterized by a composition containing (a) oxalic acid, (b) hydrochloric acid and (c) a surfactant, as an etching liquid for a transparent conductive film having an indium tin oxide (ITO) and an indium zinc oxide (IZO) as primary components.